1. Field of the Invention
The present invention relates to a photosensitive member for use in an electrophotographic process, and more particularly to such photosensitive member for forming a potential image utilizing the difference in voltage distribution resulting from the change in resistance of a photoconductive layer.
2. Description of the Prior Art
In the field of electrophotography there are already known various photosensitive members. In a most common electrophotographic process consisting of an electrostatic charging step followed by an imagewise exposure step to form an electrostatic image, the representative photosensitive member is composed of a substrate and a photoconductive layer formed thereon. The photoconductive layer can be obtained by vacuum vapor deposition of inorganic photoconductive materials such as S, Se, PbO or alloys or intermetallic compounds of S, Se, Te, As or Sb, or by a coating of a mixture of inorganic photoconductive material such as ZnO, CdS, TiO.sub.2 or PbO and an insulating binder.
The insulating binder can be composed of various resins.
The electrostatic image is generally obtained by charging the surface of the photosensitive member with a corona discharge, and then selectively dissipating the surface charge in areas exposed to a light image. The electrostatic image thus obtained is rendered visible by depositing toner particles having a charge of a polarity opposite to that of the electrostatic image, and is then transferred onto a transfer sheet. In such electrophotographic process, compacting of the apparatus is known to be difficult because of the presence of the wire and shield case for corona discharge, and of the high-voltage source therefor.
Also there are already proposed photosensitive members enabling to easily compact the apparatus, such as those disclosed in the Japanese Patents Laid-Open Sho 48-68238, Sho 51-150342, Sho 53-1027, Sho 54-61534 and Sho 54-61537. These photosensitive members are capable of forming a potential image developable with charged toner without the use of corona charging. The potential image is formed by imagewise exposure while a voltage is applied to the photoconductive layer through electrodes provided thereon, whereby a difference in voltage distribution is created between the exposed area and the unexposed area. The photoconductive layer to be employed in the photosensitive member for forming such potential image can be composed of the same materials as those employed in the photoconductive layer of the conventional photosensitive member, but differs in function from the latter.
In case of formation of electrostatic image utilizing corona charging, the exposed area of the photoconductive layer is rendered electroconductive to dissipate the charge, but, in the unexposed area the photoconductive layer has to maintain the insulating property in order to prevent charge dissipation. For this reason the photoconductive layer should be of as high resistance as possible. On the other hand, in case of forming a potential image through the difference in voltage distribution caused by a change in the resistance of the photoconductive layer, although the photoconductive layer is required to perform a distinct change in resistance in response to light, the absolute resistance need not be as high as in the formation of electrostatic image by corona charging, and the photoconductive layer is required to form ohmic contacts with electrodes, that is the photoconductive layer is such that a barrier (contact potential difference) between the photoconductive layer and the electrodes is as small as possible. This is due to the fact that the formation of a high-contrast potential image becomes difficult in case of a large barrier, since in such case the voltage drop resulting from said barrier reduces the voltage to be distributed. In this regard the Se-based photoconductive layer certainly has a high resistance, but is insufficient with respect to the ohmic contact property. Also the binder-containing photoconductive layers such as those based on CdS, CdSe, ZnO etc. have a high photosensitivity, but generate polarization under the application of a voltage due to the presence of the binder, thus resulting in gradually deteriorated contrast of the formed potential image when the voltage application is repeated. Besides the surface irregularity tends to deteriorate the image quality.
Also the photosensitive member to be employed in the formation of the potential image requires patterned electrodes such as isolated electrodes as will be explained later, and the photoconductive layer is required to have a sufficient resistance against heat and solvent involved in the vacuum vapor deposition or chemical etching in case such treatments are used in the preparation of such patterned electrodes.
The Se-based photoconductive layers and the binder-containing photoconductive layers are defective with regard to the head resistance and the solvent resistance, respectively.